This invention relates to a method of forming a magnetic body, such as a choke or transformer core, on a printed circuit board, the magnetic body, and the printed circuit board on which the magnetic body is formed.
FIG. 4A shows a conventional method for providing a ferrite core and the like on a printed circuit board, for noise control or for use in transformer and choke coil. In this case, a ferrite core is formed and split into two pieces in advance. The split ferrite core pieces 51a, 51b are then attached to a printed circuit board 3, respectively, from both sides and glued to each other via attachment holes 5, 5 created beforehand.
However, since a ferrite core is typically molded from compacted ferrite material by a sintering molding process, it is difficult to form the core to the desired measurements and shape. Thus, it is necessary to grind the sinter molded ferrite core to the desired dimensions after the core is sintered. Also, if the maximum tolerances of the sintered ferrite core are too great, the distances to adjacent electronic components on a printed circuit board must be at least as great as the tolerances, and this could prevent high density packaging. Moreover, a ferrite core which is made too thin is likely to break or get chipped when dropped or in transit.
Additionally, if the ferrite core pieces 51a, 51b are attached as above, gaps 57 are likely to appear between the respective ferrite core pieces 51a, 51b and a circuit pattern 6 on the printed circuit board 3 as illustrated in FIG. 4B. A gap 57 between the circuit pattern 6 and the ferrite core pieces 51a, 51b causes a decline in magnetic properties of the ferrite core, which increases rapidly as the gap 57 increases.
An object of the present invention is to provide a magnetic body, and a manufacturing method thereof, as an alternative to a ferrite core or similar shape, for noise control and for use in transformers and choke coils.
In order to attain the above object, the present invention discloses a method of forming a magnetic body by preparing a magnetic material by mixing and melting at least a magnetic filler and a binder containing one or more kinds of resin, setting a printed circuit board in a mold, and injection molding to or around the printed circuit board using the magnetic material, by which the magnetic body is formed to cover a circuit pattern on the printed circuit board.
According to such a forming method, the magnetic body includes resin as binder, and is thereby more resistant to shocks than ferrite alone. Owing to injection molding of the body, the molding tolerances are reduced to 10% of that of the case of sintering molding. Accordingly, there is no need to grind the magnetic body after it is formed. Also, it is possible to arrange the magnetic body close to other electronic components which are generally provided on a printed circuit board (including a flexible board). Thus, high density packaging is realized.
In another aspect of the present invention, the magnetic body is formed in such a manner that it is appressed to the printed circuit board as a result of the injection molding process. Wherein the term xe2x80x9cappressedxe2x80x9d means that a magnetic body 7 is formed as a single, unitary body without unwanted gaps therein or unwanted gaps between the magnetic body and the printed circuit board.
In this way, a decline in magnetic properties which results from gaps between the printed circuit board and the magnetic body is prevented. With injection molding, it is easy to have the magnetic body appressed to the printed circuit board.
In further aspect of the present invention, a gate for supplying the melted material to a cavity of the mold is in the form of pin gate.
In this case, a cross section of the gate becomes small and a trace of the gate on the molded magnetic body is not obtrusive, or clearly distinct. Therefore, the gate can be provided at any desired position, even when two magnetic bodies (or a magnetic body and another member) are molded adjacently.
In another aspect of the present invention, the printed circuit board has at least one through-hole in an area where the magnetic body is formed, and material is supplied to form the magnetic body on both sides of the printed circuit board via the through-hole.
In this way, the magnetic body is formed on both sides of the printed circuit board at one time, and it is not necessary to use adhesives and the like to hold the magnetic body fast to the printed circuit board.
In another aspect of the present invention, the magnetic body is formed to cover an electronic component mounted on the printed circuit board.
The covered electronic component is protected from noise. The electronic component to be covered may be a pin, resistance or condenser. Other electronic components can be covered as well.
As explained above, such a magnetic body can function to the full extent, and it does not need be ground after formed.
Additionally, the magnetic body may be formed on only one side of the printed circuit board.
On such a printed circuit as well, the magnetic body can be set adjacent to other electronic components, and high density packaging can be attained.